Magnetic sensing devices have many detection applications, including navigation, current sensing, and linear and angular position and displacement sensing. Magnetic sensors provide a non-contacting means for determining position related parameters such as shaft rotation, presence of magnetic ink, vehicle heading, etc. One of the benefits of using magnetic sensors is that the output of the sensor is generated without the use of contacts. This is a benefit because over time contacts can degrade and cause system failures.
A Hall sensor is a common magnetic sensor type having many applications including detection of the linear displacement of a target object. FIG. 1A illustrates a conventional implementation of a Hall effect linear displacement sensor 10 utilized for sensing displacement of a desired target object. Displacement sensor 10 generally comprises a Hall transducer element 2 positioned with respect to a permanent magnet 4.
In accordance with known Hall effect sensor operating principles, Hall element 2 is a magneto-electric transducer that converts a portion of the magnetic energy from permanent magnet 4 into a voltage signal that is detected and utilized such as by an automotive feedback control system. Specifically, Hall element 2 comprises a plate that is oriented in parallel with the depicted x-y plane and positioned with respect to magnet 4 such that its flat sensing surface is disposed in parallel opposition to a lengthwise surface 6 of rectangular bar-shaped magnet 4. Although not expressly depicted in FIG. 1A, it will be appreciated by those skilled in the art that in practical application, magnet 4 and Hall element 2 are coupled to mounting sites within a given system such that magnet 4 and/or Hall element 2 are linearly movable along the depicted Hall position direction with respect to the other.
As shown in FIG. 1A, Hall element 2 is generally positioned at a distance z=1.75 mm from the opposing lengthwise surface 6 of magnet 4, having a length in the y-direction of L=18.0 mm, a height in the z-direction of H=5.75 mm, and a width in the x-direction of 5.0 mm. The Hall position in the y-direction can be, for example, a 15 mm linear segment between specified endpoints, y1 and y2, which are symmetrically offset from the polarized ends of magnet 4 by approximately d=1.5 mm.
Being a linear displacement detection device, sensor 10 is designed such that a relative linear motion along the upper surface 6 of magnet 4 and Hall element 2 in the y-direction between y1 and y2 is determined and tracked in real time by Hall element 2, which detects the magnitude and polarity of the z-component, Bz, of the magnetic field produced by magnet 4. The magnitude and polarity of the orthogonal z-component component of the magnetic field detected by Hall element 2 vary in accordance with the intensity and angle of the magnetic field along the linear path of travel of Hall element 2 with respect to the length of magnet 4 between specified sensing positions.
FIG. 1B illustrates a graph depicting the varying field strength of the sensed orthogonal component, Bz, of the magnetic field as magnet 4 moves linearly in the y-direction with respect to Hall element 2. As shown in FIG. 1B, the Bz field component varies in a non-linear manner as Hall element 2 traverses a linear path parallel to the upper surface 6 of magnet 4, pitching steeply as Hall element 2 nears each of the opposing pole face ends and flattening at the magnet midpoint where Bz is substantially zero. The voltage output response from Hall sensor 2 is proportional to the observed Bz component and is therefore similarly non-linear.
The non-linear transducer response from Hall element 2 is detrimental in practice, resulting in reduced detected sensor output linearity and increased additional signal processing overhead required to linearize the detected output. This non-linearity in sensor response is particularly problematic in terms of loss of tracking accuracy at the flattened signal region.
The non-linear Hall sensor output problem is addressed by U.S. Pat. No. 6,496,003, issued to Okumura et al., which discloses a magnet shaping technique in which the magnet surface opposing the Hall element and bounded at each end by respective pole faces is arched in a manner such that the orthogonal magnetic field component varies linearly as the magnet moves linearly with respect to the Hall element. This magnet shaping technique results in secondary N—S pole pairs along the arched surface that linearize the sensed magnetic field component and thus the sensor output response. While effective for linearizing the Hall sensor output, the Okumura device is highly susceptible to linear output error resulting from variations in lateral placement of the Hall element with respect to the magnet.
In view of the foregoing, a need remains for a magnetic linear displacement sensor having a substantially linear output response while minimizing linear output error caused by variations in the construction and positioning of magnetic sensor components. The present invention addresses such a need.